1. Introduction

Diamond-like carbon (DLC) films are a kind of amorphous carbon film, wherein both the σ and π bonds due to sp

³

 and sp

² hybrid orbitals constituting diamond and graphite, respectively, are the carbon skeletons ^[1][2][3][4]. Aisenberg and Chabot conducted a series of experiments to fabricate diamond films using a carbon ion beam and confirmed the formation of diamond-like amorphous carbon films, known as diamond-like carbon. This marked the beginning of DLC research ^[5].

 hybrid orbitals constituting diamond and graphite, respectively, are the carbon skeletons [1,2,3,4]. Aisenberg and Chabot conducted a series of experiments to fabricate diamond films using a carbon ion beam and confirmed the formation of diamond-like amorphous carbon films, known as diamond-like carbon. This marked the beginning of DLC research [5].

DLC films feature high hardness, high wear resistance, low friction coefficient, high insulation, high chemical stability, high gas barrier properties, high anti-burning properties, high biocompatibility, and high infrared permeability. DLC films with flat surfaces can be synthesized at low temperature (~200 °C). Hence, they have a wide range of applications ^[6][7][8], such as electric and electronic equipment (e.g., hard disks, video tapes, integrated circuits), cutting tools (e.g., drills, end mills, razors), molds (e.g., optical parts, injection molding), automotive parts (e.g., piston rings, cam-related parts, clutch plates, pumps, injectors), optical components (e.g., lenses), plastic bottle oxygen barrier films, sanitary equipment (faucets), windows, bathtub mirrors, and decorative items. Their demand as protective films for automotive parts is rapidly increasing, particularly due to their excellent wear resistance and low friction coefficient properties ^[9].

DLC films feature high hardness, high wear resistance, low friction coefficient, high insulation, high chemical stability, high gas barrier properties, high anti-burning properties, high biocompatibility, and high infrared permeability. DLC films with flat surfaces can be synthesized at low temperature (~200 °C). Hence, they have a wide range of applications [6,7,8], such as electric and electronic equipment (e.g., hard disks, video tapes, integrated circuits), cutting tools (e.g., drills, end mills, razors), molds (e.g., optical parts, injection molding), automotive parts (e.g., piston rings, cam-related parts, clutch plates, pumps, injectors), optical components (e.g., lenses), plastic bottle oxygen barrier films, sanitary equipment (faucets), windows, bathtub mirrors, and decorative items. Their demand as protective films for automotive parts is rapidly increasing, particularly due to their excellent wear resistance and low friction coefficient properties [9].

The ratio of sp

³

 to sp

² binding in DLC ranges from ~10% to ~90%. Moreover, the hydrogen content in DLC varies from 0 to 50 at%. Robertson et al. proposed a ternary diagram ^[10] of sp

 binding in DLC ranges from ~10% to ~90%. Moreover, the hydrogen content in DLC varies from 0 to 50 at%. Robertson et al. proposed a ternary diagram [10] of sp

³

 carbon, sp

²

 carbon, and hydrogen targeting DLC films with large chemical bonding and compositional variations, which provides the understanding of DLC existence regions in the sp

³

 bond, sp

² bond, and hydrogen content axes. The diagram was constructed using 28 sets of experimental data referenced by Robertson et al. All the samples were evaluated by nuclear magnetic resonance (NMR) and elastic recoil detection analysis (ERDA). Jacob et al. proposed a fully constrained network model ^[11][12] on this ternary diagram to demonstrate the degree of clustering of the sp

 bond, and hydrogen content axes. The diagram was constructed using 28 sets of experimental data referenced by Robertson et al. All the samples were evaluated by nuclear magnetic resonance (NMR) and elastic recoil detection analysis (ERDA). Jacob et al. proposed a fully constrained network model [11,12] on this ternary diagram to demonstrate the degree of clustering of the sp

²

 phase shown as a region. Furthermore, the DLC film is classified based on the sp

³ ratio and hydrogen content cited by 19 studies ^[13]. Bewilogua et al. ^[14] and Reinke et al. ^[15] strengthened the diagrams by adding experimental data on the sp

 ratio and hydrogen content cited by 19 studies [13]. Bewilogua et al. [14] and Reinke et al. [15] strengthened the diagrams by adding experimental data on the sp

³

 ratio of hydrogenated amorphous carbon films obtained from Fourier-transform infrared spectroscopy (FTIR) spectra and confirmed that hydrocarbon films can be classified into two types, regardless of the production method. Zhang et al. created a ternary diagram comprising nanocrystalline graphite, a fused aromatic ring, and olefinic chain clusters from a survey of the Raman spectra of many DLC films; further, they classified DLC primarily based on the morphology characterized by the presence of sp

² carbons ^[16].

 carbons [16].

To date, Vetter investigated and detailed the addition of various additives (e.g., Si, F, B, Ti, Al, Mo, Co, Fe, Ni, Cu, W, Zr, Ag, Au, H, and N) in DLC films. Notably, the static contact angle of a water droplet and electric properties of DLC vary significantly, depending on the type and concentration of the third element added [17]. The film structure forms a mixing layer, which lowers the sp

³

 ratio near the substrate or forms an intermediate layer. The amorphization trajectory of the sp

² phase clustering, which ranges from crystalline graphite to amorphous carbon, has been explained by a model that changes from nanocrystalline graphite with a cluster size of ≥2 nm to a fused aromatic ring of ≤2 nm and further converts it into olefin-chain nanoclusters ^[18][19]. The hydrogen content in DLC films significantly impacts both the hardness and friction coefficient. Furthermore, the friction coefficient of DLC films grown using raw material gas with a very high hydrogen/carbon ratio (e.g., 10) is typically very low (μ = 0.003), whereas that of DLC films without hydrogen is very high (μ = 0.65) ^[20].

 phase clustering, which ranges from crystalline graphite to amorphous carbon, has been explained by a model that changes from nanocrystalline graphite with a cluster size of ≥2 nm to a fused aromatic ring of ≤2 nm and further converts it into olefin-chain nanoclusters [18,19]. The hydrogen content in DLC films significantly impacts both the hardness and friction coefficient. Furthermore, the friction coefficient of DLC films grown using raw material gas with a very high hydrogen/carbon ratio (e.g., 10) is typically very low (μ = 0.003), whereas that of DLC films without hydrogen is very high (μ = 0.65) [20].

Amorphous carbon films containing DLC films are synthesized by various methods such as chemical vapor deposition (CVD), which includes plasma-enhanced CVD ^{[21][22][23][24][25][26][27][28][29]}, electron cyclotron resonance (ECR) plasma CVD ^[30], plasma-based ion implantation and deposition (PBII&D) ^[31][32], and physical vapor deposition, which includes ionized evaporation ^[33], sputtering ^[34][35], unbalanced magnetron sputtering (UBMS) ^[36][37], ECR sputtering ^[38][39], high-power impulse magnetron sputtering (HiPIMS) ^[40][41], filtered cathodic vacuum arc (FCVA) ^{[42][43][44][45]}, ion-beam deposition (IBD) ^{[5][46][47][48]}, arc ion plating (AIP) ^[49][50], pulsed laser deposition (PLD) ^[51], and laser arc deposition ^[52][53]. Many of the DLC films produced by these methods have deviated from the DLC regions on the ternary diagrams reported to date. Therefore, redefining the DLC region on the ternary diagram, which uses DLC coatings for mechanical and electrical components, is urgently required in academia and industries. In fact, Germany made a national effort to standardize the carbon films and published the German standard VDI2840 in 2005 ^[54].

Amorphous carbon films containing DLC films are synthesized by various methods such as chemical vapor deposition (CVD), which includes plasma-enhanced CVD [21,22,23,24,25,26,27,28,29], electron cyclotron resonance (ECR) plasma CVD [30], plasma-based ion implantation and deposition (PBII&D) [31,32], and physical vapor deposition, which includes ionized evaporation [33], sputtering [34,35], unbalanced magnetron sputtering (UBMS) [36,37], ECR sputtering [38,39], high-power impulse magnetron sputtering (HiPIMS) [40,41], filtered cathodic vacuum arc (FCVA) [42,43,44,45], ion-beam deposition (IBD) [5,46,47,48], arc ion plating (AIP) [49,50], pulsed laser deposition (PLD) [51], and laser arc deposition [52,53]. Many of the DLC films produced by these methods have deviated from the DLC regions on the ternary diagrams reported to date. Therefore, redefining the DLC region on the ternary diagram, which uses DLC coatings for mechanical and electrical components, is urgently required in academia and industries. In fact, Germany made a national effort to standardize the carbon films and published the German standard VDI2840 in 2005 [54].

Therefore, in this study, 74 types of amorphous carbon films were collected, and the sp

³

 ratios and hydrogen contents of the films were evaluated. Importantly, the properties of the amorphous carbon films were each evaluated using a single near-edge X-ray absorption fine structure (NEXAFS) for the sp

³

 ratio analysis, a single 

¹³

C NMR for the sp

³

 ratio analysis, and a single Rutherford backscattering spectroscopy-ERDA (RBS-ERDA) for the hydrogen content analysis, to eliminate errors due to differences in the equipment and measurement conditions. The DLC regions were clarified on the ternary diagram based on the measured data. Subsequently, the amorphous carbon films containing DLC were classified.

The effects of sp

³

 ratio and hydrogen content on various properties of amorphous carbon were clarified by comparing and investigating the results of the sp

³ ratio and hydrogen content obtained by RBS-ERDA with those of the density, refractive index, extinction coefficient, nano-indentation hardness, visible light Raman spectrum, ultraviolet light Raman spectrum, static water-drop contact angle, and corrosion properties. Finally, the type of DLC film and its application fields were described based on the classification results and characteristic evaluation of amorphous carbon films.

 ratio and hydrogen content obtained by RBS-ERDA with those of the density, refractive index, extinction coefficient, nano-indentation hardness, visible light Raman spectrum, ultraviolet light Raman spectrum, static water-drop contact angle, and corrosion properties (Section 3). Finally, the type of DLC film and its application fields were described based on the classification results and characteristic evaluation of amorphous carbon films.